This invention relates to a diagnostic device for detecting a degradation failure of an exhaust gas sensor disposed in an exhaust gas pipe of an internal-combustion engine.
An exhaust gas sensor is generally disposed in an exhaust gas pipe of an internal-combustion engine of a vehicle in order to measure components of exhaust gas. The exhaust gas sensor outputs an air-fuel ratio of the exhaust gas. Based on this output value, a control unit of the internal-combustion engine controls a stoichiometric air-fuel ratio of fuel to be supplied to the internal-combustion engine. Therefore, when the exhaust gas sensor cannot indicate a correct air-fuel ratio due to its degradation failure, the control unit cannot perform a correct control of the stoichiometric air-fuel ratio upon the internal-combustion engine.
There are disclosed some techniques for detecting a degradation failure of such exhaust gas sensor. The Japanese Patent Application Unexamined Publication (Kokai) No. HEI 7-145751 and U.S. Pat. No. 5,325,711 disclose a technique for producing a fuel signal having a modulated rectangle waveform, detecting exhaust gas by an oxygen sensor and processing this output signal so as to determine an operating condition of the oxygen sensor.
However, the above-referenced technique uses a modulated air-fuel signal having a modulated rectangle waveform and a composite output corresponding to an oxygen level of the exhaust gas based on the modulated signal. A response, which is output upon the input of the modulated rectangle waveform containing various frequency components, tends to be influenced by noises. Furthermore, because the signal that responds to the oxygen level of the exhaust gas is influenced by an operating condition of the internal-combustion engine, in particular, an air-fuel ratio variation that may be produced during an excessive operation, it is difficult to keep the frequency of the composite output signal at a constant level. Therefore, when the sensor condition is evaluated by these outputs, evaluation precision may deteriorate. On the other hand, precision of the air-fuel ratio control is getting more important than before because of an enhanced emission control and the need for decreasing the amount of precious metals in the catalyst. Accordingly, in order to suppress an increase of the exhaust gas components due to the characteristic degradation failure of the exhaust gas sensor, it is required to improve the detection precision more than before and it is also required to suppress the increase of the exhaust gas components during the degradation detection process.
Thus, it is an objective of the present invention to provide a failure diagnostic device for an exhaust gas sensor, which enables a further improvement of precision of detecting a deterioration failure of the exhaust gas sensor as well as a minimization of an increase of exhaust gas components during a degradation detection process.